FIG. 1 depicts a diagram of the salient components of wireless telecommunications system 100 in accordance with the prior art. Wireless telecommunications system 100 comprises: wireless terminals 101-1, 101-2, 101-3, and 101-4, base stations 102-1, 102-2, and 102-3, radio network controller 111, and wireless switching center 112, interconnected as shown. Wireless telecommunications system 100 provides wireless telecommunications service to all of geographic region 120, in well-known fashion.
The salient advantage of wireless telecommunications over wireline telecommunications is the mobility that is afforded to the users of the wireless telecommunications system. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the wireless terminals are mobile, an interested party might not be able to readily ascertain the location of a terminal.
Such interested parties might include both the user of the wireless terminal and remote parties. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her own location. For example, the user might be interested in telling a remote party where he or she is. Or the user might be interested in knowing how to navigate from where he or she is to another location.
There are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of a 911 emergency call from a wireless terminal might be interested in knowing the location of the terminal so that emergency services vehicles can be dispatched to that location.
A variety of techniques are known in the prior art for locating a wireless terminal.
In accordance with one technique, a wireless terminal that is engaged in a call is deemed to be at the location of the cell that is handling the call. This technique is advantageous in that it does not require that additional hardware be added to the wireless terminal or to the wireless telecommunications system, and this means that the first technique can be inexpensively implemented in legacy systems. The accuracy of this technique, however, can be quite poor in typically-sized cells, and, therefore, this technique is generally not acceptable for applications (e.g., emergency services dispatch, etc.) that require higher accuracy.
In accordance with a second technique, a wireless terminal is located by triangulating the angle of arrival or the time of arrival of the signals transmitted by the wireless terminal. This technique is accurate to within a few hundreds of meters and is advantageous in that it can be used with legacy wireless terminals. It is disadvantageous, however, in that it generally requires that hardware be added to the telecommunication system's base stations, and this is very expensive.
In accordance with a third technique, a wireless terminal is located by a radio navigation unit, such as a Global Positioning System (GPS) receiver, that is incorporated into the wireless terminal. This technique is accurate to within tens of meters and is advantageous in that it does not require that additional hardware be added to the telecommunication system's infrastructure. Four disadvantages of GPS, however, are that it cannot be used with legacy wireless terminals that do not comprise a radio navigation unit, it often fails to work indoors, it can be easily jammed, and it can be disabled by the user of the terminal.
In accordance with a fourth technique, a wireless terminal is located by pattern-matching location-dependent signal traits measured at the terminal against a database that maps locations to predicted values of the signal traits. This technique can provide excellent accuracy, and also has the advantages of being able to work indoors, and with legacy wireless terminals.
A wireless terminal can be tracked by using any of these techniques repetitively. In general, the confidence with which a tracking system can estimate the location of a wireless terminal at any instant is a function of the frequency at which the location is determined and the mobility of the terminal. This is because the tracking system must estimate the location of the terminal for those instants between when the terminal is actually located. The longer the interval between when the terminal is located, the less confidence the tracking system can have in estimating the location at each instant during the interval. Therefore, the confidence with which a tracking system can estimate the location of a wireless terminal at any instant is a function of the frequency at which the terminal is actually located.